With his remark that the Cambridge, Mass., police acted "stupidly" in arresting Henry Louis Gates Jr., President Obama managed to extend the story by a week or more and to turn a nice little summer amusement for the political opinion industry into a "teachable moment," which means something everyone must get serious about. Obama also solidified his reputation as a foot-in-mouther almost as accomplished as his vice president. Before Gates and the police, there was his joke about Nancy Reagan conducting séances in the White House, and then an unfortunate (though very common) use of the Special Olympics as a punch line, and so on.

But Obama's rhetorical goofs usually are different from Joe Biden's momentum-mouth, just as they are different from the empty-headed nonsense of George W. Bush and the bizarre country-club-bar chatter of Bush's father. They are also different from the standard political "gaffe," which, as we know, is when a politician accidentally tells the truth. Obama's goofs are generally not a result of speaking the truth. They come from thinking things through incompletely. It turns out that the police officer who arrested Skip Gates was not necessarily acting "stupidly" and that Gates might have been doing just that. The president ultimately came up with a typically elegant formulation, describing the episode as a misunderstanding between "two good men." Wouldn't it have been better if he had just kept quiet until all the facts were in and all his thoughts were in order?

No, it would not have been better. The media, in their ill-fitting role as guardians of civility, now lecture the president on the special responsibilities of his office. His own aides no doubt shake their heads and tell one another that this is what happens when the man goes "off the reservation" -- that is, when he fails to follow the script they have written for him. And of course the Limbaughs and Gingriches of the world are so upset that they can barely contain their delight at having this stick to beat Obama with.

The rituals of umbrage that have become so big a part of our political narrative aren't just tedious. They do real harm. Very often the offense taken is completely phony, such as during last fall's campaign when Obama stood accused of insulting Sarah Palin and all of womankind by using the phrase "lipstick on a pig." Three problems here. First, the whole fuss was stagey and false. Second, it consumed valuable attention when citizens had more important subjects they should have been thinking and talking about. And third, it encouraged further fancied slights.

But even when the remark at issue is genuinely unfortunate and the offense taken isn't completely imaginary, the fuss is usually excessive and damaging. The people who declare that a president has a special responsibility not to say anything offensive have it wrong. The president has a special responsibility to address important topics and to say important things. That can't be done in a thin-skinned political culture obsessed with gaffes, and with a citizenry overly quick to take offense.

The more concerned you are to avoid saying anything wrong or offensive, the less likely you are to say anything inspiring or true. We have elected a president with a speculative mind. He wrote a book worth reading -- wrote it himself! -- even before running for president. It's interesting to hear what he thinks about various subjects -- even those that don't immediately affect his own presidency. But every teachable episode we put him through teaches him that speculation is risky. And the riskier we make it, the less of it we're likely to get.

Jokes are a slightly different category, but the dynamic is the same. The more we punish jokes that fall flat, the fewer good ones we're likely to get. Just as presidents start by chafing at the Secret Service and end up enjoying life inside the cocoon, they start by speaking their minds and gradually learn that it's safer and easier to live by the Teleprompter.

We complain about politicians who talk in pre-tested and rehearsed sound bites, but we punish anyone who strays too far into his or her own thinking.